If you want your extra leisure/auxiliary battery to get fully charged you have to have a 12V to 12V charger.

If your starter battery and your extra battery is LiFePo4 then you most certainly have to have a charger to save your alternator.

The resistant in a Lithium battery is a lot less than a Lead acid and will withdraw excessive amount of power from the alternator

I have found two solutions for Dc to Dc charger here below